According to a conventional vehicle cruise control system, there is provided an actuator which is equipped with a speed control member coupled to the throttle valve and driven by a motor to perform a vehicle speed control action, and the use of an electric motor offers the advantage of an improved control accuracy.
In such a motor-driven actuator, an electromagnetic clutch may be interposed between the motor and the speed control member to the end of selectively transmitting the rotary power of the motor to the speed control member by engaging the clutch only when a constant-speed cruise condition is produced and otherwise preventing the interference of the electric motor upon the throttle pedal. Further, to facilitate the determination of the initial condition of the speed control member, a deceleration limit switch may be provided for the purpose of detecting the initial position of the speed control member in the path for conducting the electric current for the deceleration of the vehicle to the motor so that the unnecessary rotation of the motor in the direction to achieve a vehicle deceleration can be prevented; the limit switch opens itself to shut off the deceleration drive current when the speed control member has returned to its initial position. An example of such a cruise control system is disclosed, in copending U.S. patent application No. 233,641 filed Aug. 18, 1988, and reference is made to this copending patent application for more complete disclosure of a cruise control system of the aforementioned type.
However, in such a control circuit, if the vehicle runs downhill and encounters very little running resistance, the vehicle speed may rise to such an extent that the cruise control becomes inadequate to decelerate the vehicle to a desired extent even though the control circuit supplies a deceleration drive signal to the motor in order to drive the throttle valve in the direction to decelerate the vehicle. To curb such an excessive increase in the vehicle speed, it is possible to execute a cancel operation and disable the cruise control system altogether when the vehicle speed increases beyond the set speed by more than a predetermined value, as disclosed in Japanese patent laid-open publication No. 60-163735. However, this causes a significant inconvenience to the driver because, if he steps on the accelerator pedal to accelerate the vehicle from the state of cruise control, for instance, to pass another vehicle, the control system will be canceled, and the driver has to restart the cruise control system if he wishes to regain the state of cruise control after completion of the passing.
Further, if the deceleration limit switch fails and is frozen at its open state, the deceleration drive signal produced by the control circuit is not supplied to the motor since the state of the system is identical to that when the speed control member has reached its deceleration limit position, and the excessive increase in the vehicle speed cannot be prevented.